docs: def find_one(self, filter=None):
          """
          Finds one matching query element
  
          :param query: dictionary representing the mongo query
          :return: the resulting document (if found)
          """
  
          if self.table is None:
              self.build_table()
  
          allcond = self.parse_query(filter)
  
          return self.table.get(allcond)